Various copolymers have been known as (meth)acrylic acid/alkyl (meth)acrylate ester copolymers, usable in an aqueous thickener for cosmetics or the like, a moisture-retaining agent for poultice or the like, a suspension stabilizing agent for an emulsifying agent, a suspension, or the like. For example, a copolymer prepared by reacting a specified amount of an olefinic unsaturated carboxylic acid monomer and a specified amount of an alkyl (meth)acrylate ester, of which alkyl group has 10 to 30 carbon atoms (see Patent Publication 1); a copolymer prepared by reacting a specified amount of an olefinic unsaturated carboxylic acid monomer, a specified amount of an alkyl (meth)acrylate ester, of which alkyl group has 10 to 30 carbon atoms, and a crosslinking agent (see Patent Publication 2); a copolymer prepared by reacting an olefinic unsaturated carboxylic acid monomer and an alkyl (meth)acrylate ester, of which alkyl group has 8 to 30 carbon atoms (see Patent Publication 3); and the like have been known. These (meth)acrylic acid/alkyl (meth)acrylate ester copolymers can be used in each of the applications mentioned above by usually dissolving the copolymer in water or the like, and thereafter neutralizing the solution with an alkali to prepare a neutral viscous aqueous solution having a concentration of about 0.1 to about 1% by mass.
However, in a case where an electrolyte is co-present in various raw materials and an additive constituting the manufactured articles, there are some disadvantages that a neutral viscous aqueous solution thereof has a lowered viscosity or a greater temperature dependence even at its relatively low concentration, the stability of the manufactured articles containing these aqueous solutions undesirably worsens with time, or the like. Further, there are some disadvantages that the aqueous solution also has lowered transmittance, and that a part of the copolymer is precipitated.
In particular, in recent years, in the field of cosmetics, in order to achieve differentiations among the commercial products, cosmetics having such properties as those including an active ingredient such as an electrolyte, or a mineral component in a high content, those having excellent external appearance with high transparency, and those giving a feel without stickiness are being remarked. In addition, a proposal of a thickener having properties that can expect improvements in production efficiency in various production steps has been desired, from the viewpoint of the request of reduction in production costs.    Patent Publication 1: Japanese Patent Laid-Open No. Sho 51-6190    Patent Publication 2: Japanese Patent Laid-Open No. Sho 59-232107    Patent Publication 3: U.S. Pat. No. 5,004,598